Electromagnetic device



Sept. 22, 1931. E. w. KELLOGG -7 ELECTROMAGNETIC DEVICE Filed Dec. 3, 1926 DEFLECT/ON Inventor: Edward W. Kellogg,

by His Attorney.-

Patented Sept. 22, 1931 EDWARD W. KELLOGG, OFISCHENECTAD Y, NEWLYORK,.ASSIGNOR TO GENERAL ELECTRIC l v -GOMPANY, a CORPORATION; OF NEW YORK ELECTROMAGNETIC DEVICE 7 Application-filedDecember 3,1926. j seriaino. 152,475.

The "present invention relates to "electro magnetic devices, and more particularly to the type .ofsound reproducing devices commonly known'as loud speakers. 9

One of the objects of my invention *is'to provide a vibratory system, employing a movable iron armature and a diaphragm, hav ing a low natural period of vibration.

lfVhile various" attempts have been made to secure a vibratory system having'a low natural period, it. has been difficult to ac complish such a result when employing iron armatures, owing to the 'factthat the vibrating armature is usually unstable in the magnetic fieldunlessheld by means o f"a.-stiff spring. The magnetic pull tending todisplace the armature varieswith the position of the armature in the field and increases very rapidly as the armature approaches either polepiece. If it were possible to balance/the magnetic pull. against the spring action of the armature, a vibratorysystem having a very low natural periodcould theoretically be obtained, but owing to the changing slope of the magnetic pull curve itis difficult to obtain a close balance. t"

3 While it is possibleto' obtain.a 'cl'ose'bal-v an ce between the magnetic pull and the, spring actionbythe employment of springs whose stiffness increases with deflection, I have found that a very close balancemay'be obtained with a simple spring. and a rocking armature'by restrictingthe flux in themagnetic system so that the'sum of the fluxes through the armature is practically constant. With a constant total flux the force 'deflec tion curve becomes substantially a straight line. An arrangement of this characterpossesses advantages overan arrangement in which springs of varying stiffness are employed to balancejthepull on the armature. Arlose balance between" spring stiffness and magnetic pull calls for a high degree of uniformity in magnetic strength in orderthat identicalsprings may give uniform" results in different instruments. While it is dif ficult to make'alarge'number ofmagnet-s which "will be substantiallyequ'al in strength, it is possible by the' use of saturated polepieces to obtain better uniformity in the-total amount offlux employed, since the amount of magnetic flux which a piece-of soft iron w1l1 carry 1s substantially constant.

The novel features which I believe to be 1 characteristic of" my invention are set forth with particularity in the appendedclaim's. My invention itself, however, will 'bestT'be understood from reference-to" the following s'pecifi'cationitaken in connection with the accompanying drawings in which Fig. 1 is a perspective view ofa loud speaker diaphragm and motor which embody the features of my invention Fig. 2 is a front elevation of the -magnetic system shown in Fig. 1; Fig. 3 is a curve illustrativeof the relation which ordinarily exists between the magnetic force tending to deflect a diaphragm andthe deflection of the diaphragm; Figk4'is an. explodedview of the polepieces and the mounting for the armature and the coils cooperating therewith; while Fig. 5 is an enlarged view of aportionof themotor shown in Fig. 1.; v i f Referring to the drawings, I have indicated at 1 fa cone-shaped diaphragm'which is-adapte'd to vibrate as a whole. The diahragm. may be mounted in any suitable 'manner in a cabinet having a' front panel 2pr0videdwith an opening 3.. The diaphragm isactuated by means of a motor comprising a permanent magnet 4 which is providedwi th soft iron polepieces 5 and 6. Ajfiat elongated armature 7 is mounted on a'relatively stiff spring 8 whichisattached at its opposite ends to brass or otherwise suitable non-magnetic supportin members 9 and 92 One end of the polep iece e' is secured to the permanent magnet" 4 by means of a bolt, while the other endaof the polepiece is secured to a luglO'onthe non-magnetic member9. The intermediate portion of the'pol'epiece 5 is connected to the mem her 9. As thus'arranged the pole tips 11 and 12'of the polepiecefi fitin anopening 13 between the members 9'and 9and extend toward the armature 7 as clearly indicated in Fig. 1. Pol-epiece 6 is secured in a similar manner'to the magnet 4 andto' the oppo site side of the supporting members -9 and 9', the lower end of 'thepolepiece -5 being se cured to a lug 1 1 on member 9', while the intermediate portion of the polepiece 6 is secured to member 9. Coils 15 and 16 surround the armature 7, members 9 and 9 being rounded, as indicated at 17 and 18 toaccommodate the coils 15 and 16 respectively. Armature 7 is connected to the diaphragm 1 by means of a drive rod 19 which is fiattened at the end adjacent the armature, as

indicated at 20 in Fig. 1 and on an enlarged scale in Fig. 5. When current is suppliedto the diaphragm 1 is vibrated. r

The polepieces and 6 are provided with constricted portions 21 between the'm'agnet 41 andthe-pole tips ll and 12. When the coils and 16 armature 7 is actuated and device is assembled the portions-21 become substantially saturated and thereby limit the total flux which passes through the .pole tips .11 and 12 in multiple, to a practically one end approaches pole tip 12 and the other end recedes from 11, for example, the decrease in air gap reluctance opposite 12 more than oifsets the increased reluctancev oppo-.

site 11. If an approximately constant magnetic potential were maintained between the pole tips of 5 and those of 6,-as would be the casein the absence of the saturated necks 21, the increase offlux from pole tip 12 would be greater than the decrease of fluxfrom 11. Under'these conditions the magnetic moment on the armature bears a non-linear relation to deflection or tip of the armature, as illustrated in Fig. 3. On the other hand, if the sum of the magnetic fluxes from 11 and 12 is constant, the force or magnetic moment bears a straight line relation to the deflection of the armature. Such an arrangement 1s easily stabilized and provides a close balance between magnetic pull and thespring action of the armature.

. Magnetic flux which leaks across'from 5, to 6 through paths other than across the working air gaps, tends to reduce'the effectiveness of the saturated necks in holding the sum of the fluxes in the working air gaps constant, since a reduction in reluctance of the working gaps :causes. a diversion of some of the leakage fluxrto the low reluctance gap.

It is therefore desirable that the total amount of leakageflux shall be as small as possible compared with. the useful flux. To this. end

I have designed the device shown, in such away that the working air gaps have a much lower reluctance than is usual with this type of motor. I accomplish this by making. the poleface areas, or surfaces of the le tips 11-and 12 next to-the armature, larger than has been the case in most earlier designs,

while the air gap length, orclearance between armature and pole tips is about the same as has been generally used namely from .010 to .015 inch. Y V 7 Although saturation in the armature itself would assist stabilization, I have found that where an armature having a cross-section small enoughto accomplish this purpose is employed it is too easily saturated and loss of sensitivity results. On the other hand the use ofpole-pieces with saturated necks does not in any Way impair sensitivity. In the drive having great flexibility and when employed with free edged cones the fundamental resonance is very low. For the purpose of further increasing the flexibility of the system from the standpoint of obtaining lownatural frequency the rocker arm or armature 7 may be soproportioned that the motion of-the vibrating coneor diaphragm 1 is greater than the motion of the armature in the air gaps. There is a limit to the'advantagewhich may be obtained by stepping up the'motion of the diaphragm. I have found, however, that a step-up movement which is slightly greater than 2 to 1 provides a marked advantage, although a step-up of less than 2 to 1 gives no appreciable advantage.

In addition to having a flexible rocker s pring, it is advantageous to have suflicient flexibility in the drive rod- 19 so that the stiffness of. the system. is not materially" increased by the stiffness of the drive rod. This flexibility may be obtained by flattening thedrive rod at the'end adjacent the rocker arm, as indicated at in Figs. 1 and 5.

With the drive of the type above described and a paper cone having a thickness of .006 inches and a diameter of 5%inches it will be'found that the fundamental resonance of F thesysteni (in which the cone constitutes the principal mass and the springs in the driving mechanism'constitute the principal restoring force) is below 200 cycles per second and will average about 140 cycles per second.

What I claim as new and desire to secure by Letters Patent of-the United States, is 1. In combination, a magnetic structure comprising aniarmature, a magnet, a pole piece connected to the magnet and adapted tocomplete a magnetic circuit through the magnetand-armature, said pole piece comprising a pair of pole tips of like polarity, means forvibrating-the armature, a spring adapted to hold the armature in its normal The foregoing arrangement provides a position, said armature being of such crosssection as to prevent saturation thereof, the portion of the pole piece between the pole tips and magnet being constantly magnetically saturated, whereby a straight line relation may be maintained between the deflection of the armature and the force tending to deflect it.

2. In combination, a magnetic structure comprising an armature, a magnet, means for completing a magnetic circuit through the armature and magnet, said means comprising a pole piece having a pair of pole tips of likepolarity, means for vibrating the r armature, a spring adapted to hold the armature in its normal position, said armature being of such cross-section as to prevent saturation thereof, and means for maintaining the sum of the fluxes through the armature from the pole tips of like polarity substan tially constant, said means comprising a constantly saturated portion in the magnetic circuit.

3. In combination, a magnetic structure comprising an armature, a magnet producing a force on the armature, means for vibrating the armature, a spring adapted to hold the armature in its normal position and a pair of pole pieces secured at one end to said magnet and cooperating with opposite sides of said armature, said armature being of such cross-section as to prevent saturation thereof, said pole pieces each having at the other end thereof, a pair of pole tips of like polarity, the portion of said pole pieces intermediate the ends being constantly magnetically saturated. V I i 4. A vibratory system having'a resonance frequency below 200 cycles per second and 10 including means for producing a substantially constant magnetic flux in an air gap having an area of not less than g g square inch, said means comprising a constantly saturated flux-conducting element, a diaphragm, and an armature connected to said diaphragm and vibratable in said gap, said armature being of such cross-section as to prevent saturation thereof.

In witness whereof, I have hereunto set my hand this 2nd day of December, 1926.

EDWARD W. KELLOGG; 

